1. Field of the Invention
The present invention relates to a lead frame, and more particularly, to a method for forming a plating layer of a lead frame having improved anti-corrosion properties.
2. Description of the Related Art
A semiconductor lead frame is an important element of a semiconductor package, together with the semiconductor chip. The lead frame connects the inside of the semiconductor package to the outside thereof, and supports the semiconductor chip. In general, such a semiconductor lead frame is manufactured by a stamping or etching method.
According to the stamping method, a thin plate material is formed into a predetermined shape using a press molding apparatus. The stamping method is mainly used for mass producing lead frames.
The etching method is chemical etching in which a predetermined portion of the material is eroded using a chemical, and this method is usually used to manufacture small quantities of lead frames.
In order to form a plating layer on a semiconductor lead frame manufactured by stamping or etching, a metal such as nickel (Ni) is plated on the lead frame and then palladium (Pd) or an alloy of Pd--Ni is plated thereon, where each plating step is performed by an overflow plating method.
The overflow plating is performed according to an electrolytic method or a non-electrolytic method. Of the two methods, the electroplating method is the more widely used.
In the electroplating method, a negative charge is provided to an object to be plated, such as a lead frame, to forcibly precipitate metal cations dissolved in a solution onto the surface of the object as a form of nucleus for growth. Here, the uniformity of the precipitated nucleus depends on the shape of the object to be plated, the current density, and the concentration of metal cations dissolved in the solution. When a nucleus is formed at a spot on the surface of the object to be plated, metal grows from that nucleus. Here, the nucleus growth rate is faster than the nucleus generation rate at an adjacent spot. This phenomenon can be observed by X-ray diffraction. That is, a plating layer having a predetermined crystal orientation can be seen.
In the process of forming the plating layer, gas or bubbles may be interposed between nuclei of the plating layer. Here, ions for facilitating corrosion, e.g., chlorine ions, are easily incorporated within the bubbles. In a salt spray test performed on a lead frame made of Alloy 42, which is formed of a Ni--Fe alloy, after plating the lead frame, an area of localized corrosion forms around the portions having the bubbles within 2-3 hours.
The formation of the area of localized corrosion causes corrosion of the lead frame and a decrease of the electrical conductivity, thereby resulting in a detrimental effect on the characteristics of the lead frame. Thus, after plating the lead frame, harmful gas components are removed by a thermal process, by rearranging the crystalline structure of the plating layer. As a result, the number of bubbles is minimized. In general, the thermal process is carried out in such a way that a lead frame is put into a furnace and then heated for a predetermined amount of time at a high temperature of between 600.degree. C. and 1,000.degree. C. while under a vacuum or in an inert gas atmosphere.
However, when the lead frame is processed at a high temperature of at least 600.degree. C., the surface of the plating layer of the lead frame is oxidized by contact with oxygen in the air.
When an oxide layer is formed on the surface of the plating layer by surface oxidation, a diffusion path is blocked, such that hydrogen remains within the plating layer and cannot diffuse therefrom. As a result, the plating layer corrodes due to the hydrogen molecules remaining in the plating layer, causing cracks and deterioration in solderability.